The present invention concerns apparatus for determining the operating characteristics of large antennas. More particularly, it concerns apparatus for scanning and measuring the near-field radiation of such antennas and thereby determining their far-field radiation patterns.
Near-field scanning as a method of determining the far-field properties of an antenna is well established. The following articles discuss several applications of this concept.
1. D. T. Paris, W. M. Leach, Jr., and E. B. Joy, "Basic Theory of Probe Compensated Near-Field Measurements," IEEE Trans. AP, vol 26, pp. 373-379, May 1978.
2. E. B. Joy, W. M. Leach, Jr., G. P. Rodrigue, and D. T. Paris, "Applications of Probe-Compensated Near-Field Measurements," IEEE Trans. AP vol 26, pp. 379-389, May 1978.
3. Y. Rahmat-Samii, and M. S. Gatti, "Far-Field Patterns of Spaceborne Antennas from Plane-Polar Near-Field Measurements," IEEE Trans. AP, pp. 638-648, vol 33, June 1985.
4. A. D. Yaghjian, Planar Near-Field Measurement Techniques on High Performance Arrays--Part I, Error Analysis for Nonscanning Beam Patterns, Air Force Avionic Laboratory Technical Report, AFAL-TR-75-67, July 1975.
Four distinct methods of near-field scanning have been developed. These are the planar (plane-rectangular), plane-polar, cylindrical, and the spherical scanning methods. Each has particular advantages for certain applications and has serious limitations in the maximum size of antenna that can be measured. For example, the planar method is best suited for high gain antennas, does not require the antenna under test to move, and uses a probe that scans a rectangular measurement plane immediately in front of the aperture. Probe scanners of this type are complex because the probe must be moved in two directions to cover the entire measurement (scan) plane. A planar scanner is extremely expensive to fabricate as it gets larger because of the high mechanical tolerances that must be maintained to obtain high quality measurement results. Planar scanners in use now are typically less than about 6 meters maximum dimension, although a few are larger.
Plane-polar scanners are similar in application to planar (plane-rectangular) scanners. They require the antenna to rotate about a single axis and use a probe that moves along a line perpendicular to and intersecting the antenna rotation axis. The combined motion produces a circular disc measurement surface immediately in front of the antenna aperture. This method can handle antennas large than the planar scanner but requires the antenna to be rotated. Again, as the antenna size grows, the method becomes very expensive and in fact, is not practicable for extemely large antennas. The largest antenna measured in this way was 20 meters in diameter. Also, the computation of the far field is a little more difficult because the data samples are distributed on concentric rings rather than on a rectangular grid.
In cylindrical scanning, the measurement plane is a cylinder surrounding the antenna and oriented coaxially with the rotation axis of the antenna. The probe moves along a line parallel to the rotation axis. Computation of the far fields is a little more complicated and time consuming than in planar scanning and since the method requires motion of the antenna, it is limited to the measurement of modest size antennas.
The spherical scanner is best suited for low gain antennas, requires the antenna under test to rotate about two orthogonal axes, and uses a stationary probe. The motion of the antenna relative to the stationary probe produces an apparent spherical measurement surface centered at the antenna. The method is limited to fairly small antennas because of the complex antenna rotation needed. Also, the computations required to obtain the far field are very much more complicated than the planar case and the computer running times may be prohibitive for large antennas.
Each of the four methods listed can not be applied to very large antennas of 30-100 meters or larger in size. For planar scanning, scanner construction to maintain the required tolerance is technically and financially prohibitive. For the other methods, the machinery necessary to rotate the antenna is similarly prohibitive. In addition, the superstructures required to support the measurement probe in the planar, plane-polar, and cylindrical methods can produce unwanted reflections that limit the accuracy of the measurement. In general, near-field scanning requires extremely careful mechanical alignment and the problems of maintaining this alignment are disproportionally increased with size.